The present invention relates to a radio relay method and apparatus for digital communication which applies to a heterodyne radio relay system using micro-wave or millimeter-wave radio signals.
The radio relay system is to relay a main signal and an auxiliary signal from one terminal station to another through relay stations. Two radio transmitters and two radio receivers are provided at each terminal station and two radio repeaters are provided at each relay station for back-and-forth communication. The transmitted and received radio signal includes a main signal which transmits digital information such as aural, visual, and data information, and an auxiliary signal which supervises and/or controls remotely the relay stations to maintain them in good condition at or from the terminal station. The terminal station is usually an attended station and the relay station is usually an unattended station. The auxiliary signal comprises an order-wire channel signal, a supervisory signal, and a control signal which includes a pilot signal. The order-wire channel signal is for communication between the attending persons at the terminal station and the relay station for the maintenance of the radio relay system, the supervisory signal is to supervise remotely the state of the radio repeaters from the terminal stations, the control signal is to control remotely the radio repeaters from the terminal stations, and the pilot signal is to distinguish the routes of each radio relay system. Above all, the pilot signal is very important with respect to the present invention and will be discussed in detail later. The main signal is transmitted by a radio carrier modulated by phase shift keying (PSK) modulation and the auxiliary signal is usually transmitted by modulating slightly the carrier using frequency modulation (FM).
The pilot signals are used to distinguish individually the routes of the radio relay system. FIG. 1 shows an example of two routes R1 and R2 where the route R1 consists of terminal stations T.sub.11 and T.sub.15 and relay stations R.sub.12, R.sub.13, and R.sub.14, and the route R2 consists of terminal stations T.sub.21 and T.sub.25 and relay stations R.sub.22, R.sub.23, and R.sub.24. Let us assume that the routes R1 and R2 intersect each other at the relay stations R.sub.13 and R.sub.23. A pilot signal is provided by synthesizing different low frequencies, so each route has its own pilot signal at a synthesized frequency where P.sub.1 is a pilot signal for the route R1, and P.sub.2 is for the route R2.
For the terminal and relay stations, the pilot signal is necessary to distinguish on which radio wavepath a received signal comes in, that is, whether the received signal is a desired signal or an undesired signal. The desired signal is a correct radio signal which should be received by the terminal or relay station, and the undesired signal, even though its radio frequency is the same as that of the desired signal, is an incorrect radio signal such as a radio signal which arrives at a receiving antenna of the terminal or relay station in the form of an interference wave produced by radio waves from the regular route and an adjacent irregular route. Usually, it is impossible to avoid having the undesired signal at the terminal or relay station because of antenna characteristics such as the side lobes of an antenna. This is especially true in digital communication, in which the radio relay system tends to easily pick-up undesired signals due to the wide frequency band-width required in digital communication. However, if the level of the desired signal is dominant as compared to the undesired signal level, the radio relay system is not affected by the existence of a a low level undesired signal, because, the quality of the digital communication does not depend on the signal to noise (S/N) ratio as in an analog communication. However, if the desired signal decays because of fading for example, and the undesired signal happens to become dominant the pilot signal is useful.
In FIG. 1, when the main signals are respectively transmitted from the up-line terminal station T.sub.11 to the down-line terminal station T.sub.15 in the route R1 and from T.sub.21 to T.sub.25 in the route R2, and when the radio transmission between the relay stations R.sub.12 and R.sub.13 in the route R1 is attenuated by heavy rain, an interference wave which has been produced between the radio waves of the routes R1 and R2 becomes dominant at the relay station R.sub.13, which causes the main signal in the route R2 to cut into the route R1 at the relay station R.sub.13. This results in false reception of a main signal at the terminal station T.sub.15. The radio receiver at the terminal station T.sub.15 rejects the false signal by detecting the proper pilot signal P.sub.2.
As digital communication becomes more popular, the radio spectrum for communication becomes more congested, especially in the city. Therefore, the undesired signal is becoming a very important subject which must be considered in designing the radio repeater. More details as to the received signal for the undesired signal will be explained later with respect to a block diagram of the prior art radio repeater.
FIG. 2 is a block diagram of a prior art radio repeater which consists of main and sub receiver-transmitters 50 and 51 and a supervisory-controller 52 except receiving and a transmitting antennas and their attached circuits. Both main 50 and sub 51 receiver-transmitters are hot and the one which produces an output having the best S/N ratio is automatically selected. The function of the radio repeater will be explained with respect to the main receiver-transmitter hereinafter. An input signal which arrives at an input terminal 30 from an antenna and an output from a receiving local oscillator 2 are mixed in a receiving mixer 1 and converted to an intermediate frequency (IF) signal. The IF signal is amplified by an IF amplifier 3 and fed to a squelch circuit 4. The squelch circuit 4 operates as a switching circuit so that the IF signal is transferred to a next stage when the level of the IF signal exceeds a threshold level designated in the squelch circuit 4. This switching operation can be performed as follows: and IF signal mean level provided by the IF amplifier 3 is fed to the squelch circuit through a connecting line 22; the mean level is compared with the threshold level; and the IF signal is transferred to the next stage through the squelch circuit when the mean level exceeds the threshold level and vice versa. A branch circuit is included in the squelch circuit 4 so that the IF signal which has gone through the squelch circuit 4 is separated into two signals each at half the original level, and one of the two goes to an IF amplifier 6 of the transmitter-receiver 50 and the other goes to the supervisory-controller 52. The former IF signal is amplified by an IF amplifier 6, and mixed with an output of a transmitting local oscillator 9 by a transmitting mixer 7 to convert it to a radio frequency (RF) signal. The RF signal leaves the ouput terminal 31, and is transmitted to a successive relay station through a transmitting antenna. The transmitting local oscillator 9 is modulated using frequency modulation based on an auxiliary signal which will be explained later, so that the receiver-transmitter 50 can relay the main signal along with the auxiliary signal. The later IF signal is demodulated by a demodulator 5 in which a received auxiliary signal is removed and sent to the supervisory-controller 52 through an output terminal 32 of the receiver-transmitter 50.
The received auxiliary signal which comes from the receiver-transmitter 50 is amplified by a low frequency amplifier 10 in the supervisory-controller 52, and a part of the amplified signal is fed to a filter 13 through which a pilot signal included in the received auxiliary signal is filtered out and fed to judging circuit 16. The judging circuit 16 determines whether the pilot signal is correct for the route in which its own radio repeater belongs and produces a judged signal and sends it to switching circuits 12 and 20. When the judged signal is correct, the switching circuit 12 becomes on and the switching circuit 20 becomes off. When the switching circuit 12 is on, the received auxiliary signal is fed to a remote controller 18 through a low frequency amplifier 15, and the remote controller 18 removes supervisory signals required by the relay station and inserts other supervisory signals. The inserted supervisory signals are amplified by a low frequency amplifier 19 and sent to an input terminal 33 of the main receiver-transmitter 50. The inserted supervisory signal is amplified again by a low frequency amplifier 8, and fed to the transmitting local oscillator 9. The transmitting RF carrier is modulated by the inserted supervisory signals in the form of frequency modulation. Thus, the relayed main signal and the auxiliary signal including the inserted supervisory signals can be relayed to a successive relay station with the pilot signal included in the IF signal. The same process as discussed above occurs in the sub-receiver-transmitter 51. The received auxiliary signal which comes from the output terminal 42 of the sub-receiver-transmitter 51 is fed to low frequency amplifier 11 of the supervisory-controller 52; this signal is also dealt with using the same process as mentioned above, and the switching circuit 12 compares both signals amplifiers by the low frequency amplifier 10 and 11 and selects the signal having better S/N ratio. The inserted or selected supervisory signal is also applied to an input terminal 43 of the sub-receiver-transmitter 51 to modulate the RF carrier. The RF signals from the receiver-transmitters 50 and 51 are compared and an RF signal having the highest level is transmitted to the successive relay stations.
When the desired signal decays and there is no undesired signal which is received by the radio repeater, the squelch circuit 4 in the receiver-transmitter 50 turns off, so that there is no output of the received auxiliary signal at the output terminal 32. Then, the judging circuits 16 and 17 in the supervisory-controller 52 turn the switching circuit 12 off so that the connection between the low frequency amplifiers 10 and 15 is off. As a result the inserted supervisory signal is fed to the low frequency amplifier 19, and the switching circuit 20 is turned on, and a generated normal pilot signal from a pilot signal oscillator 21 is also fed to the low frequency amplifier 19. The inserted supervisory signal and the generated correct or normal pilot signal are fed to the transmitting local oscillator 9 and modulate the transmitting RF carrier. The inserted supervisory signal includes information indicating that the relay station in question has not received the desired signal, and the terminal station can recognize that there is trouble at a point in the path before this relay station.
The explanation above is with respect to the receiving state at the radio repeater when the desired signal is dominant over the undesired signal or when the desired signal is decayed and the radio repeater does not receive any radio wave. However, the following state or situation sometimes occurs. The radio repeater receives both signals, a desired signal and an undesired signal, the level of the undesired signal is lower than that of the desired signal but higher than the threshold level of the squelch circuit. In this state, when the desired signal level is sufficiently high in comparison with that of the undesired signal, the operation of the radio repeater is normal. However, once the level of the desired signal decays due to fading for example, the level of the undesired signal becomes dominant, so that a main signal and an auxiliary signal from the undesired signal become dominant. As a result, a received pilot signal is incorrect, so the judging circuit 16 turns the switching circuit 12 off and the switching circuit 20 on. Then, the inserted supervisory signal is sent to the transmitting local oscillator 9 with the generated correct pilot signal. On the other hand, the IF signal which includes an incorrect pilot signal goes through the squelch circuit 4 and is fed to the transmitting mixer 7 through the IF amplifier 6. Consequently, the received incorrect pilot signal and the generated correct pilot signal are mixed in the transmitting mixer 7 and sent to a successive relay station.
When such a mixed pilot signal arrives at the terminal station, since each pilot signal consists of different low frequency signals, the terminal station is thrown into confusion as to the pilot signal. This causes the terminal station to reject the whole received signal. This means that the terminal station can not receive the supervisory signal from the relay station, in other words, there is no way to find out which station is experiencing fading and which relay station is in trouble. This has been a problem of the prior art radio relay method and apparatus for the digital communication.